I think I know why one would think it's gray.
The metallic gray paint is not only reflecting light back into the camera, but there are times where you'll notice it's reflecting light onto the side surface of the nose. This is not always the case; sometimes the side of the nose is picking up light from off-to-the-side lighting.
Sometimes it's both.
What one needs to do in this case is to take into account that the helmet is 3D, and to wait for Vader to turn his head slightly away from a suspected light source to see if the shine goes away.
In the above scene Tantive IV, Vader is talking to one of his officers. When his head is angled more into the camera (left) you can clearly see that the side of the nose is black. There is no disputing that. But once he turns more towards the officer, the gloss of the paint picks up lighting.
If you also look very carefully, even if this were paint, it does not explain why paint that is typically solid in its highlight areas would fade out. The reflection in this shot is slightly dimmer and suggests more a reflection of light from the adjoining cheek surface's metallic gray which itself is picking up some overhead light. You can tell Vader was walking under light that is particularly brighter to this side of the face as you can observe how bright the light is on the tubes and side of the mouth on the right screen capture versus the more muted look in the left screen capture.
Both screen captures, again, are from the same scene, just mere seconds apart from each other.
